I've fallen deep into that rabbit hole, and the definition of 'best' really hinges on how dark you want to go. If you're after a series where the romance is genuinely unsettling and the vampirism is more curse than glamour, you can't skip 'The Night Huntress' world by Jeaniene Frost. It starts with 'Halfway to the Grave'. The central relationship between Cat and Bones is brutally intense, built on power struggles and a lethal partnership. It's not cozy; the stakes feel real, and the supernatural politics are gritty.
For something that leans even more into gothic despair and morally ambiguous dynamics, 'Black Dagger Brotherhood' by J.R. Ward is a classic, but the early books especially have that dark, visceral edge. The romance in 'Dark Lover' is wrapped in violence and a very raw, almost primal energy. Some find the later books get a bit formulaic, but the first few definitely deliver on the dark, adult promise.
A newer series that grabbed me is 'The Serpent and the Wings of Night' by Carissa Broadbent. It’s often shelved as romantasy, but the atmosphere is relentlessly grim, the trials are vicious, and the central vampire romance is born from betrayal and necessity. It has that 'only one can survive' tension that makes every interaction charged and dangerous.